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Tools, Tips, and Workflows

Breaklines, Part 5—Applying Breaklines to Enforce Varying Elevation

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LP360 Basic Edition ArcGIS

Page 1 of 8

QCoherent Software LLC

LP360 Standard Edition Windows www.LP360.com

LP360 sUAS Edition GeoCue 256-461-8289 (phone)

LP360 Advanced Edition 256-461-8249 (fax)

Lewis Graham

August 2013

Revision 1.0

In the last edition of LP360 News, we discussed the creation of 3D breaklines for enforcing situations

where the elevation must be a constant along the breakline. The most common example of this

applications is “water body flattening” such as lakes and ponds. In this final installment of the breakline

series, we will consider the case of varying elevation along the breakline.

Recall that, for our purposes, a 3D breakline is a vector that has an elevation value (Z) associated with

each vertex. Generally, 3D breaklines can be divided into two categories – those with the same

elevation for each vertex (used for flat water bodies, for example) and those with the ability to store a

different elevation value for each vertex (a down-stream flow polyline, for example).

In this edition, let’s look at a varying Z example such as the edges of a road or, more commonly, a

downstream flow. As with flat water bodies, a common method of collecting breaklines is to use heads-

up digitizing from an orthophoto for the X, Y (planimetric) aspects of the construction and to probe the

LIDAR (or, more generally, point cloud data) for the Z value. Unlike the constant Z flat water body

breaklines, we have to store the Z for varying elevation breaklines at each vertex. This means that these

breaklines are always represented by 3D features.

One very useful way to locate and visualize drainage is via a contour display. Figure 1 shows an area of

drainage. I have filtered the point cloud to ground points only and visualized as a triangulated irregular

network (TIN). The area of drainage has been roughly sketched in blue using an ArcGIS annotation. Note

that the contours “point” upstream due to the depression formed by the drainage. Thus, you can use

the ground class along with an examination of contours to identify drainage location and direction.



Lewis Graham Page 2 of 8 QCoherent Software LLC August 2013 www.LP360.com

Figure 1: Visualizing Drainage via Contours

There are at least two approaches to digitizing the actual drainage breakline. The first is to identify the

drainage purely from a two dimensional top view (“Map View”). After digitizing, the Z value can be

obtained from the point cloud surface via probing. In LP360, we call this process of probing the point

cloud data for the elevation value “Z Conflation.” The problem with this approach is that, even with

contours, it can be difficult to correctly identify the lowest point of drainage in the planimetric sense.

More importantly is enforcing a hydrological model. We need to ensure that draining is strictly

monotonically decreasing as we move downstream from vertex to vertex. Fortunately, LP360 contains

advanced tools for properly enforcing downstream constraints.

Figure 2 depicts the dialogs in LP360 for digitizing a downstream constraint. The dialog to the left in

Figure 2 contains overall “Z conflation” settings where general parameters are configured. Note that I

have set this to “Downstream Constraint.” The dialog to the right is used for configuring the specifics of

the conflaton method; in this case, the downstream constraint. All conflation methods in LP360 allow

you to change the vertex spacing, if desired. The general choices are:




Pure Drape – This creates a vertex at each point where the breakline intersects the edge of the

triangulated irregular network (TIN) created from the point cloud

Create vertex every X map units – inserts new vertices at the user-specified spacing

Get Z for existing vertices – do not modify the existing vertex locations.

Figure 2: The Downstream Constraint Tool in LP360

The remainder of the dialog settings on the right in Figure 2 are related to error monitoring. The

downstream constraint will, after the vector digitizing is complete, adjust the assigned values such that

the monotonicity is maintained (that is, each subsequent vertex, as one walks down the breakline, has a

Z value lower than the immediately previous vertex).




Most topology systems use the sense (direction) of a vector to determine the downstream direction.

Usually, moving in the direction of increasing vertex ID is considered the downstream direction. The first

section of the error management of the downstream constraint dialog provides some self-evident

options for how this is handled.

The second section of the error portion of the dialog allows you to monitor how much a particular

vertex is moved by the system to maintain the monotonic constraint. The distance between the surface

derived from the point cloud and the Z of the vertex is referred to as the “Delta.” We take this measure

to be the Adjusted Z (the final vertex Z) minus the surface (or Original) Z. Thus, an adjustment of a vertex

beneath the surface creates a negative Delta. If desired, the Delta of every vertex can be stored in the

Measure (M) value of the vertex, assuming you created a 3D geometry feature with M values.

Finally, summary statistics for each feature can be stored in the Attribute table. The computed values

include:

Minimum – The largest deviation below the TIN surface

Maximum – The largest deviation above the TIN surface

Mean – The mean of the deviations

Standard deviation – the statistical standard deviation of all of the Deltas of the vector

The actual digitizing of a breakline in LP360 is performed using both the Map View and the Profile View.

The Map View provides the planimetric location, whereas the Profile View allows you to visualize the

vertical placement.

A general technique that I find useful for drainage centerline is to turn on contour display in the Map

View and identify the drainage that I want to digitize using the drainage induced distortions in the

contours. It can be useful to roughly digitize these locations using the Sketch tools in ArcGIS. I then set

up for digitizing in the Profile/Map View. This generally is initiated by beginning a breakline sketch in the

Map View and then pressing the “P” key to synchronize the Profile View. This is depicted in Figure 3.

Note the red box in the Profile View. Here I have boxed in the intersection of the projection from the

Map View (vertical white line) with the ground points (orange in the Profile View of Figure 3). This

vertical line is “dropped” from the Map View into the Profile View and tracks the cursor as you adjust

the position in the Map View. Thus, one would adjust the location of the vertex of the breakline by

moving to a point such that the dropped line crosses the surface model at the lowest point.

There are a number of keyboard activated tools within the LP360 product for assisting with the

collection of breaklines, such as nudging vertices up and down in the vertical direction, overriding the

point cloud value and so forth. I encourage you to peruse the help and user documents to become

acquainted with this rich set of collection tools.




Figure 3: Digitizing a breakline using the Profile View

The digitized breakline is depicted in Figure 4 as the green line. The blue sketch line was my initial

approximation of the breakline location based on the contours. Note that the vertex property page is

displayed to the right. Note first of all, that the Zs are monotonically decreasing (the vector extends

from the lower left as the high point to the upper right as the low end). Secondly, note the M values in

the fourth column of the table. These values represent how far above (positive M value) or below

(negative M value) our algorithm had to move the vertex to maintain monotonicity.




Figure 4: The digitized breakline (green)

Finally, in Figure 5 is depicted the enforced downstream constraint breakline (the purple vector). Note

that the effect of a downstream constraint on the contours will be quite subtle if the contour is drawn in

the correction location. This is because the point cloud itself is a very accurate representation of the

drainage topography, and hence you would not expect a significant change.




Figure 5: The enforced downstream constraint

We support several other variable Z breaklines in LP360. Among these are:

Double line drains (“river flattening”) – This is used for wider streams and rivers where the

overall elevation in the direction of flow is monotonically decreasing, but the banks

perpendicular to the direction of flow must be at the same elevation (otherwise the flow

direction would be cross channel rather than down channel).

Retaining Wall – The retaining wall tool allows you to digitize parallel lines with a slight

displacement. Each line can be conflated using a different algorithm. The primary use is along




walls where using the high points for one line and the low points for the parallel line will create

a model of the wall. This model can be used in subsequent breakline enforcements.

This concludes the LP360 News series on breakline collection and enforcement within the LP360

product. As you can see, this is a very rich area of LP360 and a reason that LP360 is one of the most

widely used tools in the point cloud industry for adding and visualizing hydrological (and other forms) of

constraints. I encourage you to thoroughly review our help files and tutorials if your work involves

breakline collection. The tool you need is most likely in our collection! In addition, I encourage you to

attend our training sessions that will be held in Huntsville, Alabama, USA from September 24 through

September 26 of 2013. We will be devoting one of the afternoon sessions to the intricacies of breakline

collection.

Documents

Tools, Tips, and Workflows Breaklines, Part 5 Applying Breaklines …kb.geocue.com/wp-content/uploads/2015/01/Breaklines-Part... · 2015. 3. 6. · Tools, Tips, and Workflows Breaklines,